Thermomechanical behavior at the nanoscale and size effects in shape memory alloys
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aria L. Nó Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apdo 644, 48080 Bilbao, Spain
Christopher A. Schuhb) Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 13 February 2011; accepted 23 July 2011)
Shape memory alloys (SMA) undergo reversible martensitic transformation in response to changes in temperature or applied stress, resulting in the properties of superelasticity and shape memory. At present, there is high scientific and technological interest to develop these properties at small scales and apply SMA as sensors and actuators in microelectromechanical system technologies. To study the thermomechanical properties of SMA at micro and nanoscales, instrumented nanoindentation is widely used to conduct nanopillar compression tests. By using this technique, superelasticity and shape memory at the nanoscale have been demonstrated in micro and nanopillars of Cu–Al–Ni SMA. However, the martensitic transformation seems to exhibit different behavior at small scales, and a size effect on superelasticity has been recently reported. In this study, we provide an overview of the thermomechanical properties of Cu–Al–Ni SMA at the nanoscale, with special emphasis on size effects. Finally, these size effects are discussed in light of the microscopic mechanisms controlling the martensitic transformation at the nanoscale.
I. INTRODUCTION
Recently, there has been growing interest in the possible use of shape memory alloys (SMA) in micro and nanoscale structures and devices, e.g., as sensors or actuators in microelectromechanical systems (MEMS). With a growing worldwide market in excess of 100 billion dollars, MEMS and nanoelectromechanical systems (NEMS) constitute a new paradigm of technological development for the present century and have already found usage as sensors and actuators across numerous industrial sectors.1 The development of multifunctional and smart materials2 is converging with miniaturization technologies, enabling a new generation of smart MEMS (SMEMS). Among the different smart materials targeted for use in SMEMS, SMA have attracted considerable interest3,4 because they offer the highest output a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/ This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2011.291 J. Mater. Res., Vol. 26, No. 19, Oct 14, 2011
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work density, about 107 J/m3,5 and exhibit specific desirable thermomechanical effects such as superelasticity and shape memory, due to the reversibility of their thermoelastic martensitic transformation.6 MEMS components that exhibit superelasticity, one-way or two-way shape memory, could enable a new generatio
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